Preparation of pure metallic carbides



Patented May 15, 1 951 PREPARATION OF PUREv METALLIC CARBIDES Holbert E.Dunn, Grafton, and William T. Bradley, Carnegie, Pa., assignors toVanadium Corporation of America, New York, N. Y., a corporation ofDelaware Application July 19, 1947, Serial No. 762,074

11 Claims.

This invention relates to a process of purifying metallic carbidescontaining graphltic carbon, whereby metallic carbides which aresubstantially free from graphltic carbon and other impurities can beproduced.

The metallic carbides, for example, vanadium carbide, titanium carbide,tungsten carbide, zirconium carbide, columbian carbide, tantalumcarbide, chromium carbide and molybdenum carbide and some of thecarbides of non-metallic elements, as for example, boron carbide, areprepared by fusing an oxide of metallic element and carbon in suchproportions that the solidified regulus contains graphitic carbon inaddition to its combined carbon. It also contains nitrogen and otherimpurities. The number and 'size of the graphite particles are dependentupon conditions existing at the time of their formation. They areusually finely dispersed therein and intimately mixed with the carbidebase which prevents an easy separation. On the other hand, some of themore stable oxides often are not completely reduced by ordinarypyrometallurgical methods, even in the presence of a large excess ofcarbon. Such unreduced oxides may appear as impurities in the carbide.Quantitatively, however, free carbon is usually the major impurity incarbides prepared by such pyrometallurgical methods.

Graphitic carbon and other impurities are undesirable in the applicationof metal carbides, as for example, where the metal carbides are to beused in the production of sintered carbide cutting tools, dies and wearresisting parts. Attempts have been made to avoid the presence ofgraphltic carbon by decreasing the amount of carbon mixed with themetallic oxide in the melting operation but this causes large losses inthe metal content.

Laughlin Patent 2,310,964 describes a method of purifying metalliccarbides which involves attrition grinding the metallic carbide in aball mill to 325 mesh particle size, agitating the attrition groundcarbide with a soap solution, allowing the carbide to settle and pouring011 the supernatant liquor. Such method has met with no significantsuccess for it removes only a part of the graphltic carbon and causesappreciable losses of the metallic carbide. The difiiculty of separatinggraphltic carbon from metallic carbides by such settling and decantingmethods arises from the fact that particles of titanium carbide of 44microns diameter (325 mesh) will settle through water at a rate of only0.256 centimeters per second faster than graphite particles of the samesize. The patentee also came to the conclusion that froth flotationmethods are completely ineiiectual in removing graphitic carbon fromtitanium carbide.

Contrary to the conclusions reached in the above referred to patent, wehave found that substantially all graphitic carbon may be removed frommetallic carbides without appreciable loss of the carbide itself andthat other undesirable impurities are substantially eliminated. Aspreviously stated, the particles of graphltic carbon are dispersedthroughout the metallic carbide and in order to release them from themetallic carbide, it is necessary to reduce the particle size of thecarbide to approximately the size of the graphltic carbon. Usually thismeans that the metallic carbide must be reduced to a particle size of325 mesh although in some cases grinding to a particle size of mesh willrelease substantially all of the graphite. Considering the case where itis necesary to grind to -325 mesh in order to release the graphiticcarbon, if the usual attrition grinding procedure such. for example, asgrinding in a ball mill is resorted to, it will be found that thegraphite particles cannot be separated satisfactorily from the metalliccarbide particles by a froth flotation step. The attrition grinding ofthe metallic carbide in a ball mill results in a large proportion ofultra-fine particles of both graphitic carbon and metallic carbide.Although metallic carbides have a higher specific weight than graphiticcarbon, when the two are ground to very fine particle size, they bothtend to respond to froth flotation treatment in a similar manner so thatthey both tend to go into the froth rather than allowing the graphiticcarbon particles to go off in the froth and the metallic carbideparticles to be recovered as a sink product. Attrition grinding alsocauses the ultra-fine graphitic carbon particles to become smeared onthe particles of metallic carbide and to cause them to be carried off inthe froth.

In accordance with our invention the size reduction of the reguluscontaining metallic carbide and graphitic carbon is carried out in suchmanner as to produce a minimum of fines and a minimum of coating of themetallic particles by graphltic carbon. In other words the comminutionof the regulus is carried out in such manner as to keep the size of theindividual particles passing through a screen of given size as near aspossible to the maximum size which will pass through that screen.Overgrinding of the metallic carbide and graphite is detrimental to theseparation of these materials by the flotation step and in our processis avoided. Thus if it is necessary to reduce the size of the regulus to-325 mesh in order to release substantially all of the graphitic carbonfrom the metallic carbide so that the two may be separated by frothfiotation, the method employed in reducing the regulus to 325 mesh issuch as to produce a minimum of particles which are considerably finerthan 325 mesh. Our process involves two different types of comminutionprocesses in preparing the regulus for froth flotation. The first typeof comminution step is a crushing operation which is characterized by amaximum of impact breaking and a minimum of attrition grinding. Oneexample of such crushing method is stage roll crushing as shown inFigure 1 and described more particularly hereinafter. Another method ofcrushing which results in a minimum offines is crushing innan air sweptball mill in which the air sweeps away the fines rather than to allowthem to be continually ground finer. Another type of crushing whichproduces a minimum of fines is the employment of an air swept ringrollcrusher in which the air'likewise sweeps away the fines rather thanallowing them to be ground finer. In carrying out our method, the stageroll crushing or its equivalent is carried out until the regulus hasbeen reduced to a particle size necessary to release a substantialportion of the graphitic carbon from the metallic carbide. This step isusually carried out until the particle size has been reduced to between10 and 65 mesh, say for example, mesh. By this procedure a considerableportion of the comminution necessary to reduce the particle size to -325mesh has been accomplished by means which produces a minimum of fines.After the regulus has been reduced to 10 to 65 mesh by the stage rollcrushing or its equivalent, it is subjected to attrition grinding to 325mesh particle size or to other particle size necessary to releasesubstantially all of the graphitic carbon from the metallic carbide. Inview of the fact that a considerable portion of the comminuation of theregulus has been carried out in such manner as to avoid the formation offines, the product of 325 mesh particle size contains a much smallerproportion of ultra fines than would be the case if the wholecomminution had been carried out by attrition grinding in a ball mill.The smaller proportion of ultra fines enables good separation of thegraphitic carbon from the metallic carbide by froth flotation.

In the accompanying drawings which illustrate certain preferredembodiments of our invention,

Figure 1 is a diagrammatic illustration of a preferred method of stageroll crushing to produce a product of 20 mesh by down size containing aminimum of fines;

Figure 2 is a fiow sheet illustrating a preferred method of treatingtitanium carbide containing not over about 8% of graphite; and

Figure 3 is a fiow sheet illustrating a preferred -tratedgirrFi gure3.Referring to Figure 2, ti-

method of treating titanium carbide containing*= over about 8% ofgraphite.

In view of the difficulties presented in the removal of contaminatingimpurities introduced in the pyrometallurgical process of smeltingmetallic carbides, all possible care should be exercised 4 in suchproportions that the solidified regulus resulting from the smeltingoperation contains more than about 8% graphitic carbon. A'greatadvantage of employing such metallic carbide lies in the fact thatas-th'e graphitic carbon content is increased above about 8%, thenitrogen content drops sharply to a very low value and the ratio ofmetal to combined carbon in the purified carbide approaches more nearlythe stoichiometric proportions for pure metallic carbide.

In carrying out our process, lumps of the crude carbide or regulus arecrushed in a manner characterized by a maximum of impact breaking and aminimum of attrition grinding to a particle size necessary to release asubstantial portion of the graphitic carbon from the metallic carbide.This usually involves crushing to a particle size between about 10 and65 mesh and may be carried out in the manner'illustrated in Figure 1,which method results in crushed carbide of 20 mesh by down sizecontaining a minimum of fines, the method being commonly known as,stageroll crushing. Referring to that figure, lumps of metallic carbide of 1to 3 inch size or larger are crushed in a jaw crusher l to produceparticles inch by down size. These particles are fed between crushingrolls 2 set at inch opening. The material which passes the rolls 2 isscreened on a 10 mesh screen 3. The 10 mesh or finer material 4 passingthrough the screen is collected and the over size material 4a is passedthrough a set of rolls 5 set at V inch opening. The material coming fromrolls 5 is screened through a 10 mesh screen 6 and the material 1passing through the screen is collected. The over size material 8 ispassed through rolls 9 set at 1 inch (10 mesh) opening. The materialpassing through the rolls 9 is added to the 10 mesh material which haspassed through the screens 3 and 6. The 10 mesh material is passedthrough rolls Ill set at 3 inch opening (20 mesh) and then screenedthrough a 20 mesh screen II, the over size material l2 being returned tothe rolls III. In this manner a 20 mesh by down material is producedwhich contains a minimum of fines.

In carrying out this process it is preferred that the size reduction perpass be limited to a ratio of about 2:1. This ratio is maintained in theprocess of Figure 1 as will be seen from the fact that the successiverolls are set'at A inch opening, A; inch opening, inch opening and inchopening. If the reduction per pass is greater than about 2:1, thetendency to produce objectionable amounts of fines is increased.

After the crude carbide has been reduced to 10 to 65 mesh with theproduction of a minimum of fines, the subsequent steps of the processwill depend on the amount of graphitic carbon in the crude carbide, andthe degree of purification required. If the crude carbide contains lessthan about 8% of graphite, it is preferred to carry out the process asillustrated in Figure 2. If it contains over about 8% of graphite, it ispreferred to employ an additional flotation step as illustanium carbideor other metallic carbide which has been crushed to 20 mesh size by themethod illustrated in Figure l is subjected to attrition grinding withwater in a ball mill for a length of time such that 95% is finer than325 mesh. The aqueous pulp of this finely ground carbide at a pulpdensity of 33% solids is subjected to froth flotation. The frothflotation may be carried out with the liquid basic, acidic or neutral.but

.best results are obtained if the hydrogen ion con- 'centration isadjusted to pH 7.0 or less. If basic materials, such as lime or soda,were used in prosons of overall economy and simplicity of operation. Ourinvention is not limited to the use of the particular reagents mentionedbut on the" contrary, other reagents alone or in combination, and eithernatural or syntehtic, may be employed, such as those sold under thetrade names Aerosol, Tarol and Pentasol. One of the most eflicient ofthe reagents is, however, pine oil.

the pulp in the presence of a suitable reagent or usually mechanicallyagitated and air under pressure is admitted. As the froth develops abovethe pulp, it is continuously removed until that which remains becomeswhite and devoid of free carbon. The frothing reagents are replaced byadding them as needed. Their consumption does not usually exceed 0.50pound per ton of carbide treated, and often is as low as 0.10 pound perton.

The floated material is predominately graphite containing some flnemetallic carbide, but this metallic carbide is usually only a very smallfraction of the total treated, while the product remaining in theflotation machine is the metallic carbide substantially free ofgraphite.

Hydrochloric acid is then added to the pulp of metallic carbides todissolve impurities such as silicon, iron and other metals. The solutionis flltered and the filtrate is discarded. The filter cake is dried andscreened to break up agglomerates, which results in titanium carbide of325 mesh by down size which is substantially free from graphite andother impurities.

Figure 3 illustrates a method of purifying metallic carbide containingabove about 8% graphite. According to this method, crude titaniumcarbide is crushed to 20 mesh by down size according to the methodillustrated in Figure 1 or by another equivalent method which produces aminimum of fines. The 20 mesh by down crude carbide is subjected to afroth flotation step which removes a part of the graphite in the froth.The semi-cleaned metallic carbide is filtered and then attrition groundin a ball mill for a time sufficient that 95% passes through a 325 meshscreen. The pulp is subjected to froth flotation which removessubstantially all of the graphite and the pulp of cleaned metalliccarbide is treated with acid as described in connection with Figure 2 inorder to remove silicon, iron and othermetals. Thereafter the treatmentis in accordance with the process shown in Figure 2. The

graphite floats from the first and second flotation steps are combinedand filtered and dried. The

Crude titanium carbide containing 1.29%

graphite was crushed to 20 mesh by stage roll crushing as illustrated inFigure 1 and the 20 mesh carbide was ground dry in a ball mill to 96%325 mesh. Two thousand grams of the carbide was added to approximatelyseven thousand ccs. of water in a Denver Sub-A flotation machine(2000-gram size). This titanium carbide aqueous pulp was agitated for 15minutes, then 10 drops of pine oil was added and the agitation continuedfor 5 more minutes; Air was then admitted and the froth whichimmediately began to form was removed by continual scraping until thebubbles became small, white and free of graphite attachments. The airand agitation were then stopped, the flotation machine drained, and thecontents filtered. The resulting filter cake was dried and analyzed.This treatment reduced the graphite to 0.31% while the recovery oftitanium carbide was 97% of that in the feed.

Example 2 Crude vanadium carbide containing 4.54% graphite was crushedto 20 mesh by the stage roll crushing method illustrated in Figure 1 andthe 20 mesh carbide was attrition ground in a ball mill to 200 mesh. Twothousand grams of the ground carbide was added to 7 liters of water in aDenver Sub-A 2000-gram size, flotation machine. The vanadium carbideaqueous pulp was agitated for 15 minutes. Twelve drops of pine oil wasadded and the agitation continued for 10 minutes. The air was thenadmitted and the froth removed until the bubbles formed were free ofgraphite, after which the agitation was stopped and the flotationmachine emptied. The contents were filtered, and the filter cake wasdried and analyzed. The cleaned vanadium carbide now contained only0.46% graphite while the vanadium carbide recovery was 87.81% of that inthe feed.

Example 3 Crude zirconium carbide was crushed to 20 mesh by the stageroll crushing procedure illustrated in Figure 1 and then ground dry in aball mill to 97.5% 325 mesh. One thousand grams of the ground zirconiumcarbide analyzing 82.05% zirconium and 3.13% graphite was added to a1000-gram, Fagergren flotation machine with 2900 cc. of water andagitated for 10 minutes. Five drops of Tarol" was then added and thefroth continued to be removed until it was free of graphite, then themachine was drained and the contents flltered, dried and sampled. Thiscleaned zirconium carbide contained only 0.26% graphite and 85.12%zirconium, which represented a zirconium recovery of 95.55% of thatcontained in the flotation feed.

Example 4 Crude titanium carbide was crushed to 20 mesh by the stageroll crushing procedure shown in Figure 1 and the 20 mesh carbide waswet ground in a ball mill at 50% solids pulp density to 98% 325 mesh.One hundred and sixty pounds of the ground titanium carbide containing5.19% graphitic carbon was added to a Denver unit ,cell of 9 cubic feetcapacity, the machine fllled to theiiesired height with water and thepulp at 32.5% solids was agitated 10 minutes. Two and one-half cubiccentimeters of pine oil was added, a few drops at a time, and air wasadmitted to form a froth. The froth was continually removed by theautomatic front paddle on the flotation machine. After approximately anhour had elapsed, the froth became free of graphite and the flotationwas stopped. The

76 cleaned titanium carbide after filtering and drying contained 0.11%graphitic carbon and 75.00% titanium which represented 81.2% of thetitanium contained in the feed.

Example This example illustrated the process as applied to thepurification of metallic carbides contaming a relatively largepercentage of graphitic carbon, the process employing two flotationsteps. Titanium carbide containing 11.73% graphitic carbon was crushedto --20 mesh by the stage roll crushing procedure illustrated in Figure1 and 150 pounds of the 20 mesh titanium carbide was added to a Denverunit cell of 9 cubic feet capacity. The machine was filled with water tothe operating height and the pulp at 31.1%

' was then added to a ball mill with sufllcient water to make a 50%solids pulp density and ground to 96% 325 mesh, then drained from theball mill and added to the same flotation machine and refioated at 27.8%solids using 2.5 cc. of pine oil as frothing agent and 1250 cc. ofconcentrated hydrochloric acid for pH control. The froth was removeduntil it was clean, then the pulp was drained from the machine andfiltered. The filter cake was dried and sampled. The cleaned titaniumcarbide now analyzed only 0.22% graphite and the titanium carbiderecovery was 76.98% of that in the feed.

In carrying out the froth flotation step to remove graphitic carbon inthe froth, pulp compositions ranging from 10 to 50% solids can beemployed. In carrying out this separation in the smaller size flotationcells, such for example as batch type laboratory machines, pulpscontaining 10 to 25% solids may be used, the preferred working range forthese smaller units being 20 to 25% solids. For the large size cells,such as those used in commercial practice, pulps containing 30 to 35%olids are preferred. Thus while the full utilizable range is 10 to 50%solids, best results are secured between the limits of 20 to 35%, with30 to 35% most desirable, depending somewhat upon the type of equipmentused.

It will be seen from the description of the process hereinbefore giventhat the combination of stage roll crushing to produce particles of 10to 65 mesh while producing a minimum of fines when combined with theattrition grinding step to reduce the particle size to 325 mesh resultsin a finely ground crude carbide from which the graphite may be floatedoff in the froth whereby a good separation of graphitic car-bon andmetallic carbide may be obtained. Ordinary methods of grinding such asball mill grinding to 325 mesh, do not result in a product from whichthe graphitic carbon may be reliably floated 011' in a froth flotationstep with acceptable commercial efllciency. Accordingly, we have deviseda comminution procedure which renders feasible the separation ofgraphite from metaliic carbides by a simple froth flotation.

The invention is not limited to the preferred embodiments or examples,but may be otherwise embodied or practiced within the scope of thefollowing claims.

We claim:

1. The method of purifying a metallic carbide regulus containingcombined carbon and graphitic carbon, which comprises crushing theregulus to a maximum particle size in the range of 10 to 65 mesh by aseries of breaking operations each of which is followed by a removal ofparticles of less than the maximum size to release a substantial portionof the graphitic carbon from the metallic carbide, attrition grinding toa maximum particle size of 150 mesh to release substantially all of thegraphitic carbon from the metallic carbide, forming an aqueous pulp ofthe ground carbide including a frothing agent, agitating the pulp toform a froth, separating the froth containing graphitic carbon from theliquid pulp while continuing agitation of the pulp, and recoveringmetallic carbide from the pulp.

2. The method of purifying a metallic carbide regulus containingcombined carbon and graphitic carbon, which comprises crushing theregulus to a maximum particle size in the range of 10 to 65 mesh by aseries of breaking operations each of which is followed by a removal ofparticles of less than the maximum size to release a substantial portionof the graphitic carbon from the metallic carbide, attrition grind- 7ing to a maximum particle size of 325 mesh to release substantially allof the graphitic carbon from the metallic carbide, forming an aqueouspulp of the ground carbide including a frothing agent, agitating thepulp to form a froth, separating the froth containing graphitic carbonfrom the liquid pulp while continuing agitation of the pulp, andrecovering metallic carbide from the pulp.

3. The method according to claim 2, wherein the attrition grinding to amaximum particle size of 325 mesh is carried out by a ball millingoperation.

4. The method according to claim 2, wherein the crushing of the regulusto a maximum particle size in the range of 10 to 65 mesh is carried outby a stage-roll crushing operation.

5. The method according to claim 2, wherein the crushing of the regulusto a maximum particle size in the range of 10 to 65 mesh is carried outby an air-swept ball milling operation.

6. The method according to claim 2, wherein the crushing of the regulusto a maximum particle size in the range of 10 to 65 mesh is carried outby an air-swept ring-roll crushing operation.

7. The method according to claim 2, wherein the crushing of the regulusto a maximum particle size in the range of 10 to 65 mesh is carried outby a stage-roll crushing operation andwherein the attrition grinding toa maximum particle size of 325 mesh is carried out by a ball millingoperation.

8. The method according to claim- 2, wherein the aqueous pulp of groundcarbides has a solids concentration of about 10 to 50%.

9. The method according to claim 2, wherein the pulp is leached withacid after froth flotation and before recovering metallic carbide fromthe pulp.

10. The method of purifying a metallic carbide regulus containingcombined carbon and graphitic carbon, which comprises crushing theresulus to a maximum particle size in the range of 10 to 65 mesh by aseries of breaking operations each of which is followed by a removal ofparticles of less than the maximum size to release a substantial portionof the graphitic carbon from the metallic carbide, forming an aqueouspulp of the ground carbide including a frothing agent, agitating thepulp to form a froth, separating the froth containing graphitic carbonfrom the liquid pulp while continuing agitation of the pulp, subjectingthe non-floating residue to attrition grinding to a maximum particlesize of 150 mesh to release a further quantity of graphitic carbon fromthe metallic carbide, forming an aqueous pulp of the ground carbideincluding a frothing agent, agitating the pulp to form a froth,separating the froth containing graphitic carbon from the liquid pulpwhile continuing agitation of the pulp, and recovering metallic carbidefrom the pulp.

11. The method of purifying a metallic carbide regulus containingcombined carbon and graphitic carbon, which comprises crushing theregulus to a maximum particle size in the range of 1 0 to 65 mesh by aseries of breaking operations each of which is followed by a removal ofparticles of less than the maximum size to release a substantial portionof the graphitic carbon from the metallic carbide, forming an aqueouspulp of the ground carbide including a frothing agent, agitating thepulp to form a froth, separating the froth containing graphitic carbonfrom the liquid pulp while continuing agitation of the pulp, subjectingthe non-floating residue to attrition grinding to a maximum particlesize of 325 mesh to release substantially all of the graphitic carbonfrom the metallic carbide, forming an aqueous pulp of the groundcarbide, including a frothing agent, agitating the pulp to form a froth,separating the froth containing graphitic carbon from the liquid pulpwhile continuing agitation of the pulp, acid leaching and recoveringmetallic carbide from the pulp.

HOLBERT E. DUNN. WILLIAM T. BRADLEY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,161,011 Breerwood June 6, 19392,183,500 Crawford Dec. 12, 1939 OTHER REFERENCES Taggart-I-Iandbook ofMineral Dressing, 1945, section 4, pages 93 to 98, section 12, page 95.

1. THE METHOD OF PURIFYING A METALLIC CARBIDE REGULUS CONTAININGCOMBINED CARBON AND GRAPHITIC CARBON, WHICH COMPRSES CRUSHING THEREGULUS TO A MAXIMUM PARTICLE SIZE IN THE RANGE OF 10 TO 65 MESH BY ASERIES OF BREAKING OPERATIONS EACH OF WHICH IS FOLLOWED BY A REMOVAL OFPARTICLES OF LESS THAN THE MAXIMUM SIZE TO RELEASE A SUBSTANTIAL PORTIONOF THE GRAPHITIC CARBON FROM THE METALLIC CARBIDE, ATTRITION GRINDING TOA MAXIMUM PARTICLE SIZE OF 150 MESH TO RE-